If you still haven't bought the tickets, make sure you buy it BEFORE the day to avoid peak traffic and unfortunate incidents such as service outage or payment failures.
Mumei is performing in Day 1's concert and holo*27 stage, during Day 2 of the Fes. Please also note that you buy tickets for the individual shows, as there are three in total over the course of two days. So, if you wish to watch all concerts with Mumei's preformance you would need to buy BOTH Day 1 and holo*27's tickets. If you want to see all the EN girls then you need either all three or just Day 1 & 2.
See the line-up page for details.
Please note that you can only choose to buy the ticket for watching via Streaming+ if you use the English page, together with having to pay a gaijin tax of 650 yen for each ticket. (6500 yen in JP page vs 7150 in EN)
Venue(physical) ticket sales have ended.